The present invention relates to electromechanical devices for turning an output shaft to a desired rotational position in response to an electrical position signal input to the device. Devices of this type are known to employ a direct current motor driving a gear train to provide a desired torque with speed reduction of the rate of rotation output shaft as it is rotated to the desired position. In such an arrangement, it is known to provide a feedback potentiometer which is connected to be rotated by the output shaft to vary the resistance in a circuit. The variable resistance may be used to balance a Wheatstone bridge circuit, and thus null the output of the bridge circuit, when the desired potentiometer resistance is obtained by rotation of the output shaft. Typically, in known feedback type servoactuators, the Wheatstone bridge is unbalanced by an amount of an electrical resistance representative of a predetermined desired position of the output shaft. The motor is then energized and the output shaft rotated until the feedback resistor balances the Wheatstone bridge circuit: the nulled output of the bridge circuit is utilized to de-energize the motor thereby stopping rotation of the output shaft at the desired position. It is also known to use a comparator for detecting a predetermined voltage from the potentiometer and utilizing the comparator to output to effect breaking of the motor drive circuit.
Feedback servoactuators are employed in numerous applications where it is desired to rotate a shaft to a desired position in response to an electrical command position signal. One such application of a feedback servoactuator is that employed in automotive passenger compartment heating and cooling systems where the position of the air blend door, or vane, is electrically controlled for apportioning the flow of heated and cooled air in the discharge ducting.
Heretofore, feedback servoactuators employed for automotive blend door positioning have utilized separate wire wound or carbon resistive potentiometers for varying the feedback resistance in response to rotation of the output shaft. This has necessitated a geared or separate drive coupling of the output shaft to the moveable wiper of the potentiometer. Separate couplings from the output shaft to the potentiometer wiper have proved to be costly and cumbersome in manufacturing. Known potentiometer coupling arrangements have introduced a source lost motion between the output shaft and the potentiometer wiper in the coupling and have thus caused error in the shaft positin feedback signal.
Furthermore, servoactuators of the aforesaid type for automative air temperature control systems generally utilize a lower power small fractional horsepower motor having a high motor shaft speed and a speed reducer for providing fractional RPM and relatively high torque at the output shaft for the desired output function. Where low torque drive motors having high shaft speeds are employed, with substantial speed reduction, it has been found quite difficult to provide such a servoactuator that was not prohibitively costly. The costliners has been due in part to the need for metal gears to provide the substantial speed reduction and yet provide sufficiently quiet running to be acceptable for automotive passenger compartment heating and cooling system service.
In providing servoactuators for automotive passenger car service, as described above, it has long been desired to find a way of providing a servo for which the motor could be electrically supplied power directly by solid state switching devices, such as FETs with low current draw at low voltages of the order of 12-14 volts supplied by the on-board vehicle battery/alternator system. In order for the solid state switching devices to provide the motor command signals directly, the motor must necessarily be of very low wattage power consumption. In order to develop the required servo output torque and slow rate of rotation for automotive applications, it has been found necessary to utilize a small very high shaft RPM motor with a multiple stage speed reducer to provide the desired output torque. In providing such a high RPM motor input, speed reduction and high torque precision position seeking output, it has been found difficult to provide the quiet running capability required for automotive passenger compartment service application.
Thus, it has long been desired to find a way or means of providing a reliable consistant potentiometer feedback signal in a position seeking servoactuator which is simple in construction without employing separate couplings between the output shaft and the potentiometer which have been found to be a source of error in the position feedback signal. It has also long been desired to provide an output rotary position-seeking servoactuator which is low in manufacturing cost and utilized a motor capable of direct control by low voltage solid state switching and yet provides quick response and is quiet running.